Vascular abnormalities and development of hypoxia in microscopic melanoma xenografts

Background: Studies investigating the oxygenation status and the development of hypoxia in microscopic tumors are sparse. The purpose of this study was to measure the extent of hypoxia in microscopic melanoma xenografts and to search for possible mechanisms leading to the development of hypoxia in these tumors. Methods: A-07, D-12, R-18, and U-25 human melanoma xenografts grown in dorsal window chambers or as flank tumors were used as preclinical tumor models. Morphologic and functional parameters of vascular networks were assessed with intravital microscopy, and the expression of angiogenesis-related genes was assessed with quantitative PCR. Microvessels, pericytes, and the extent of hypoxia were assessed by immunohistochemistry in microscopic tumors by using CD31, alpha SMA, and pimonidazole as markers, and the extent of radiobiological hypoxia was assessed in macroscopic flank tumors. Results: Macroscopic R-18 and U-25 tumors showed extensive hypoxia, whereas macroscopic A-07 and D-12 tumors were less hypoxic. R-18 and U-25 tumors developed hypoxic regions before they reached a size of 2-3 mm in diameter, whereas A-07 and D-12 tumors of similar size did not show hypoxic regions. The development of hypoxic regions was not caused by low vessel density, but was rather a result of inadequate vascular function. Inadequate vascular function was not caused by low vessel diameters or long vessel segments, but was associated with poor vascular pericyte coverage. Poor pericyte coverage was associated with the expression of eight angiogenesis-related genes. Conclusions: Two of the four investigated melanoma models developed hypoxic regions in microscopic tumors, and the development of hypoxia was associated with poor vascular pericyte coverage and inadequate vascular function.